The present invention relates to cathode ray tube display devices with unnecessary radiating magnetic field reducing means which reduces an unnecessary radiating magnetic field due to a magnetic field leaking from deflection coils wound around a deflection yoke of a cathode ray tube such as a display monitor, and more particularly to measures to be taken against an unnecessary radiating magnetic field from a vertical deflection coil.
Unnecessary radiating magnetic fields which will be produced around a CRT display device, Visual Display Unit (VDU), are said to be reduced to within a predetermined value because detrimental effects to human bodies and/or electronic devices present in the unnecessary radiating magnetic fields are required to be prevented. Especially, for an unnecessary radiating magnetic field in a low frequency band below 400 kHz, there is guidelines, for example, called TCO which stipulates that an extremely low frequency magnetic field (ELMF) of 5 Hz-2 kHz and a very low frequency magnetic field (VLMF) of 2-400 kHz should be restricted to within 200 and 25 nT!, respectively, at each of a measuring point on the periphery of a circle having a radius of 50 cm! at whose center the VDU is situated and a measuring point distant by 30 cm! from the CRT tube face.
Since a main source for generating an unnecessary radiating magnetic field of such a low frequency band is the deflection yoke, many display devices are proposed which reduce the unnecessary radiating magnetic field to within a stipulated value, for example, by providing means in the deflection yoke for generating a magnetic field which cancels the unnecessary radiating magnetic field.
As a specified example, an unnecessary radiating magnetic field reducing device disclosed in JPA-3-289029 is provided with a first cancel coil connected in series or parallel with the horizontal reflection coil, and a second cancel coil connected in a series or parallel with the vertical deflection coil, the first and second cancel coils being disposed around the deflection yoke, so as to generate a canceling magnetic field opposite to an unnecessary radiating magnetic field from the deflection yoke to cancel the unnecessary radiating magnetic field.
FIG. 10 shows this conventional unnecessary radiating magnetic field reducing device. Reference numeral 1 denotes the deflection yoke; 2 a deflection coil bobbin; 3 the vertical deflection coil; and 4 a magnetic ferrite core; 55a, 55b, 56 each the cancel coil which cancels the unnecessary radiating magnetic field. The cancel coils 55a and 55b connected to the horizontal deflection coil and the cancel coil 56 connected to the vertical deflection coil 3 generate magnetic fields which cancel unnecessary radiating magnetic fields generated from the horizontal and vertical deflection coils depending on the respective horizontal and vertical scanning frequencies, respectively, to reduce the unnecessary radiating magnetic fields.
The setting positions of the cancel coils are distant from the unnecessary radiating magnetic field generating sources in the conventional unnecessary radiating magnetic field reducing device, so that there are large unnecessary radiating magnetic fields around the VDU which cannot be canceled by the cancel coils.